This application relates to intrusion detection systems, and, in particular, to systems with a centrally located antenna and a transmission line extending around the perimeter to be protected. The system encompasses signal processing circuits which calculate and accumulate incremental changes related to phase and magnitude of the received energy and use the accumulated values as indications of the presence of an intruder.
The use of leaky coaxial cables in intrusion detection systems is known. As described in Canadian Pat. No 1,014,245 and the corresponding U.S. Pat. No. 4,091,367 a pair of leaky coaxial cables can be used to identify an intruder crossing the cables. One of the cables is connected to a transmitter and the other to a receiver. Another system, as disclosed in U.S. Pat. No. 3,794,992, issued Feb. 26, 1974 to Gehman discloses an intrusion detection system in which a central VHF transmitting antenna is coupled to buried sensing antennas which surround the perimeter. Gehman teaches a series of separate identical sensing antennas consisting of a single insulated wire of size between number 10 to number 30.
One of the limiting factors in the use of either the pulse or CW leaky coaxial cable sensor is the effect of a changing environment. For example, changing soil moisture content for a buried leaky cable sensor can have a detrimental effect, as the permitivity and conductivity of the soil also changes, therefore causing the return signal to alter in magnitude and phase. In practice, these effects have been separated from legitimate targets by means of high pass filtering. The success of this operation depends on the speed of the environmental effects relative to the lowest speed target. While this has been successful for many applications, the environmental effects are still the major source of nuisance alarms.
In a leaky coaxial cable sensor employing a transmit cable and a receive cable there is a change in the relative phase of the received signal as a target walks along the transducer cables. This can be demonstrated by plotting the incremental in-phase signal as a function of the incremental quadrature signal as the target walks along the transducer. The resulting plot is circular and the distance the target moves to complete 360.degree. of relative phase is equivalent to half a wavelength at the cable velocity of propagation. It should be noted that since the velocity of propagation inside the cable is typically 79% that of free space then the wavelength is also reduced by 79%.
If all targets walked parallel to the transducer cables and within the detection zone, detection could be based on target induced change in relative phase and be much more immune to environmental effects as several cycles of phase rotation take place prior to detection. While rapid environmental changes cause some phase change they do not normally produce the same amount of phase change as a human target. In the system of this invention the detection circuit effectively tracks the target, and in doing so it uses more target information to reduce nuisance alarms due to the environment.